gralloc: add flush and reread for locked buffers

When a buffer is locked (mapped to the CPU) by two or more clients,
there is no good way to manage a reader/writer relationship. There are
no requirements for how the writes should propagate to the readers.
Clients must unlock and relock to be sure writes are flushed.
They must unlock and relock to get the lastest copy of the buffer.

This patch adds explicit flush and reread commands to help readers
and writers synchronize without having to unmap and remap the
buffer.

Bug: 136316517
Test: TODO

Change-Id: I10d3de1b0e46c4f3b50dc34aea653701933638a9